The data center is at the core of today’s business, and fiber-optic connectivity is the fabric, carrying vital data to drive critical business processes and providing connectivity to link servers, switches and storage systems.
Data center designers have two high-level choices when it comes to fiber types: multimode fiber and singlemode fiber. In this chapter, we’ll discuss the development, deployment and advantages of each fiber type, as well as the connectors that pull it all together.
Multimodefiber-thelow-costplatform
Multimode fiber (MMF) continues to be the predominant fiber type deployed in Enterprise Data Centers today. It was initially deployed in telecom networks in the early 1980s. With a light-carrying core diameter about six times larger than singlemode fiber, MMF offered a practical solution to the alignment challenges of efficiently getting light into and out of the cabling.
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Today, MMF is the workhorse media for data centers because it is the lowest-cost way to transport high data rates over the relatively short distances in these environments. MMF has evolved from being optimized for multimegabit-per-second transmission using light-emitting diode (LED) light sources to being optimized to support multigigabit transmission using 850 nm vertical cavity surface emitting laser (VCSEL) sources, which tend to be less expensive than their singlemode counterparts.
This leap in performance is reflected in the classifications given by the standards bodies. OM1 and OM2 represented the earlier MMF types with low modal bandwidth and very limited support for higher speed optics. OM3 and OM4 represent the newer, laser-optimized MMFs that are typically installed in data centers today.
The following table provides examples of some of the current data center applications and the maximum channel lengths over different fiber types.
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Introducingwidebandmultimodefiber(WBMMF)
OM3 and OM4 provide very high modal bandwidth at 850 nm, the predominant wavelength that can be efficiently supported by VCSEL transmitters. To support an increase in performance over a single pair of multimode fibers, additional wavelengths need to be transmitted alongside 850nm, achieved via a new technology — shortwave wavelength division multiplexing (SWDM). Because the modal bandwidth of OM3 and OM4 fibers were specified for laser operation at 850 nm only, a new specification for optical fiber was required. Many data center managers are now considering wideband multimode fiber (WBMMF), which optimizes the reach of SWDM transmission that delivers four times more information with the same number of fiber strands over practical distances. Being optimized to support the additional wavelengths required for SWDM operation (in the 850 nm to 950 nm range), WBMMF ensures not only more efficient support for future applications across the data center fabric, but also full compatibility with legacy applications because it remains fully compliant to OM4 specifications.
WaveDivisionMultiplexing
By the middle of 2017, the journey to standardization of WBMMF cabling was complete, having been recognized by ISO/IEC and TIA standard bodies. The OM5 designation was adopted for inclusion of this new cabled optical fiber Category in the 3rd Edition of the ISO/IEC 11801 standard. Once again, CommScope led the market in generation standards development as well as product availability and was one of the first manufacturers to deliver a commercially available OM5 end-to-end solution, with the distinctive lime green color that is also being recognized by standards bodies. Well ahead of standards ratification, CommScope introduced the LazrSPEED OM5 Wideband solution in 2016, knowing that the support of higher data throughput using low cost optics, is exactly what Data Center Managers require to enable next generation networks today and in the future.
And the future of OM5 is very bright indeed. At the end of 2017, the IEEE agreed to initiate a project to define next generation multimode transmission using short wave division multiplexing, the transmission technology that OM5 was designed to support.
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Singlemodefiber:Supportinglongerdistances
Designed with a much narrower core, singlemode fiber (SMF) is the technology of choice for longer reach applications in the data center, such as extended runs in the fabric between leaf and spine switches, spine and routers, and into the transport network to connect data centers in different locations. SMF provides higher bandwidth and does not have the modal dispersion limitations inherent in MMF. For this reason, SMF is used in applications where support for higher and next-generation bandwidths can be absolutely guaranteed to be supported. This makes it a perfect media of choice for hyperscale and service provider data center owners to
Very large data centers as well as hyperscale data centers typically deploy SMF to connect multiple halls and extended equipment zones using a centralized connects architecture at the MDA. They typically use a dedicated optical distribution frame (ODF). Deploying an ODF can help to ensure that cables are kept to an optimum length for transmission, while equipment zones and other data halls can be quickly and efficiently patched to one another with the minimum disruption to service and networking equipment.
Singlemode fiber also enables duplex transmission at higher speeds because it is able to transport multiple wavelengths, thus reducing fiber counts. It is anticipated that one of the 200GE and 400GbE applications will utilize four-pair parallel optics over SMF, taking advantage of the lower overall system cost that parallel optics can offer. The PSM4 multisource agreement (MSA) also defines a four-pair transceiver for 100G
Fiberconnectorspullitalltogether
Fiber connectors have evolved along with fiber-optic cabling, driven by increasing fiber density. The duplex LC connector emerged during the early 2000s as the predominant two-fiber type and remains so today. While the evolution of the duplex connector was underway, array connectors (parallel optics) were also emerging. First deployed in public networks, the multifiber push-on (MPO) connector has become a preferred choice for rapidly deploying cabling into data centers. The compact form of the MPO allows 12 or more fibers to be terminated in a compact plug, occupying the same space as a duplex LC. The MPO’s high density enables installation of preterminated, high-strand-count cables, while eliminating the time-consuming process of field installing connectors on site.
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Bringingoptionstoachangingenvironment
The data center is a complex environment, comprising of a wide range of equipment and technology manufactured by many different companies. Ever-increasing bandwidth and line rates have led to optical fiber being the preferred technology to enable higher speeds. To ensure proper operation and maximum efficiency of the data center networks, optical transceivers of the same type must be interchangeable and interoperable so that replacements and upgrades can be performed quickly and easily, without the need to replace or modify other network equipment.
The solution is a multisource agreement (MSA) — an agreement among multiple manufacturers to make equipment consistent and interchangeable by defining a common physical form for various devices and components. In the case of data center connectivity, there are MSAs that cover both the specification and implementation of the optical transceivers made by various manufacturers.
The phenomenal growth in data, voice and video has triggered the need for higher and higher speeds in the data center and across data centers. This has driven the standards bodies to develop higher application speeds, which in turn have driven the need for new MSAs. Per the latest version of the Ethernet Roadmap, there are currently seven new applications in progress, most of which involve fiber optics.
There are now many different MSAs which reflect the variety of applications we see in the data center:
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ExamplesofopticalMSAs
The optical transceiver MSA environment is very dynamic, with numerous MSAs — too many to list in this publication. These MSAs cover everything from form factor, application (standard, prestandard or proprietary), maximum power consumption, fiber connector type, strand count, wavelength and cable reach. Examples of current and future MSAs are shown below:
The clear trend in the development of new MSAs has been toward both higher speeds and increased densities. Higher speeds are the result of new applications standards that specify higher line rates. Higher densities have largely been driven by technology advances which enable the transceiver to make use of lower power, which allows for smaller packaging. As shown above, the larger physical sized MSAs are designed to accommodate higher power transceivers, while reduced power transceivers can make use of smaller MSAs, thus more ports or higher density communication hardware.
Fortunately, each of the data center cabling standards (TIA 942, ISO/IEC 11801-5 and CENELEC 50173-5) have standardized on two optical connectors for use in the data center: the LC for single or duplex applications and the MPO for applications requiring more than two fibers. This has simplified the fiber connectivity as the MSAs that are relevant in the data center environment also have made use of the LC and MPO connectors. And while the standardization of connectors has helped to simplify cabling, it has also become very important to provide very flexible, agile connectivity that can accommodate the ever-increasing speeds and the higher densities that are being driven by higher densities at the equipment faceplate.
To accommodate the rapid growth of cloud-based storage and compute services, traditional enterprise data centers are evolving, adapting their current architectures to accommodate new, agile, cloud-based designs. These new enterprise architectures are different from the traditional three-layer switching topologies, resembling "warehouse scale" facilities and designed to support many different enterprise applications.
Data center designers are using leaf-spine architecture to achieve an optimized path for server-to-server communication that can accommodate additional nodes as well as higher line rates as the network grows. The meshed connections between leaf and spine switches — often referred to as the network “fabric” — allow applications on any compute and storage device to work together in a predictable, scalable way regardless of their physical location within the data center.
Future-readynetworkfabrictechnology
In response to the demand for lower costs and higher capacities, new fabric network-based systems that support cloud compute and storage systems are becoming the architecture of choice for today’s data centers. The performance of the network fabric is well suited to establishing universal cloud services, enabling to-any connectivity with predictable capacity and lower latency.
These fabric networks can take many forms, from fabric extensions in a top-of-rack deployment, to fabric at the horizontal or intermediate distribution area, to a centralized architecture. In all cases, consideration must be given to how the physical layer infrastructure is designed and implemented to ensure the switch fabric can scale easily and efficiently.
The fabric has inherent redundancy, with multiple switching resources interconnected across the data center to help ensure better application availability. These meshed network designs can be much more cost-effective to deploy and scale when compared to very large, traditional switching platforms.
The design of high-capacity links is more complex since the number of links and link speeds are both increasing. Providing more data center capacity means pushing the limits of existing media and communication channel technologies. As shown below, the Ethernet Alliance Roadmap shows existing application standards as well as predictions of application rates beyond 1 terabit/second. This will further challenge complexity as application speeds move from duplex transmission to parallel transmission. With the advent of new technologies such as shortwave WDM (SWDM), wideband multimode fiber (WBMMF), BiDi transmission, CWDM and more efficient line coding, it is anticipated that the transition to parallel optics will be delayed.
EthernetSpeed
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PossibleFutureSpeed
The combination of SWDM and WBMMF provides the opportunity to extend the use of multimode technology, which continues to be the most prevalent fiber technology deployed in data centers. Engineered solutions make complex fabric network designs easy to design, implement and manage. terminated high-performance systems support generation network media and duplex and multifiber modular applications, while reducing deployment management time and expense.
